The present invention relates to an electro-optic display system that is especially useful for mirrors in the automotive vehicle environment.
Rearview and sideview mirrors for a vehicle such as an automobile are standard equipment which allow the driver to observe surrounding traffic. Although such mirrors are very helpful, they can become a nuisance at night due to glare reflected into the eyes of the driver by the headlights of following vehicles. To solve this problem, in the past rearview mirrors were adjustable to two positions to provide day and night modes. The day mode position operates as a normal mirror, reflecting substantially all of the light to the driver. In the night mode position, the mirror is tilted to reflect only a portion of the incoming light, thus reducing objectionable headlight glare. Although this type of mirror alleviates the problem, it must be either manually adjusted by the driver or an expensive motor drive and sensing system is employed.
The two-position adjustable rearview mirror can be replaced by an automatically adjusting electro-optic mirror, the structure and operation of which are known in the art. An electro-optic mirror has an electro-optic medium, the ability of which to transmit light can be increased or attenuated depending on the strength of an applied electrical field. The electro-optic medium typically consists of either electrochromic or polymer dispersed liquid crystal (P.D.L.C.) material. If electrochromic material is used, the medium is normally in a clear state and is darkened by application of an electric field. If P.D.L.C. material is used, the medium is normally in a darkened state and is cleared by application of an electric field. The electro-optic medium is located in the mirror between front and rear transparent conductive layers to which the electric field is applied. A reflective layer is positioned behind the rear conductive layer such that its reflective surface faces forwardly towards the electro-optic medium. The mirror so formed includes an electrical control circuit to automatically adjust the transmissivity of the medium depending on the intensity of light sensed by photodetectors. If a photodetector senses "day" conditions, then, depending on the type of electro-optic material used, an electric field is or is not applied across the electro-optic medium such that the medium is in a clear state so that substantially all of the light is reflected by the mirror surface to the driver. However, if the photodetector senses "night" conditions and a second photodetector detects glare from a following vehicle, then, depending on the type of electro-optic material used, an electric field is or is not applied across the electro-optic medium such that the medium is selectively darkened, attenuating its ability to transmit light, such that undesired glare is substantially reduced.
It is known in the prior art to equip electro-optic mirrors with display systems to display information to the driver such as the time of day, ambient temperature, compass headings, and the like. Frequently, such mirror displays use illuminated displays that are viewed through the electro-optic medium or through a window created in the electro-optic medium. An alternate approach is disclosed in U.S. Pat. No. 5,189,537 to O'Farrell. The display system disclosed by this patent has an indicia defined by dielectric material positioned between the electro-optic medium and a conductive layer such that the electric field in the vicinity of the indicia is minimized. This causes the contrast between the indicia and the electro-optic medium to be maintained regardless of whether the mirror is in the day or night mode. This display system is limited, however, because it only provides a permanent, unchangeable display of the indicia pattern.
Thus, there exists a need for an electro-optic mirror display system capable of contrastingly displaying changing information, in either day or night conditions, and without the use of an illuminated display.